The Status of Mars Climate Change Modeling

Robert M Haberle
Space Science Division
NASA-Ames Research Center
Moffett Field, CA 94035.
Email: [email protected]

Mike Carr and Bruce Jakosky have reviewed the evidence that the climate of Mars has changed throughout its history. In this talk, I review where we stand in terms of modeling these climate changes. For convenience, three distinct types of climate regimes are considered: very early in the planet's history (> 3.5 Ga) when warm wet conditions are thought to have prevailed; the bulk of the planet's history (3.5 - 1 Ga) during which episodic ocean formation has been suggested; and relatively recent in the planet's history (< 1 Ga) when orbitally-induced climate change is thought to have occurred.

1. Early Mars Greenhouse Models. The valley networks and highly eroded landforms of the late Noachian period imply liquid water was stable at that time. The most plausible way to produce such conditions is to invoke the greenhouse effect of a more massive CO2/H2O atmosphere than the one we see today. During the 1970's and 80's 1 and 2-dimensional models were developed which showed that global mean temperatures could reach 273K in the presence of a less luminous sun if the atmosphere contained between 1-5 bars of CO2 - an amount consistent with estimates of the planets volatile inventory. The lifetime of such an atmosphere against weathering has been estimated to be 10M - 500M years and could be sustained for comparable times by "hot spot" volcanism or impact cratering. However, these early greenhouse models are flawed because they do not account for atmospheric CO2 condensation which can greatly retard the greenhouse effect. Furthermore, atmospheric evolution models are unable to arrive at current conditions from a massive early CO2/H2O atmosphere. At the present time, there is no resolution of this dilemma. The geological evidence suggests warm and wet conditions, but the climate models are unable to show how this can occur. Some possible solutions include a brighter early sun, the presence of reduced greenhouse gases, and a scattering greenhouse effect. But these all have difficulties. It is also possible that the models are missing some important physics, or that networks and eroded landforms actually formed in cooler environments than have been suggested. But without more work and/or data, we cannot determine which of these possible solutions is correct.

2. Episodic Ocean Formation. Baker et al. (1991) have suggested that a variety of surface features (young networks, eskers, sedimentary deposits, shorelines) could be explained by episodic ocean formation throughout Mars' history. These oceans would form in days to years as the result of flooding associated with volcanic activity in the Tharsis region. The oceans are expected to contain large amounts of dissolved CO2 which would come out of solution and go into the atmosphere. The released CO2 and H2O would increase the greenhouse effect which would then force even more CO2 into the atmosphere from the regolith and polar cap reservoirs. Baker et al. estimate that as much as 4 bars could be added to the atmosphere by this mechanism. Thus after ocean formation the climate would warm and a hydrologic cycle would develop. Weathering would draw down atmospheric CO2 and the ocean would eventually be returned to the ground water system. The processes associated with this scenario are poorly understood as no modeling has appeared in the literature. However, Gulick et al. have recently submitted a paper to Icarus in which they address the duration and thermal environment of an ocean-induced climate event. They find that a 1-2 bar pulse of CO2 occurring anytime during the past several billion years is capable of raising global mean temperatures to 240-250K for 10's to 100's of million years. Such an increase could drive a limited hydrological cycle and possibly explain the younger valleys and putative glacial features. However, many details remain most notably the possibility for multiple ocean forming events since the first event would convert a large amount of CO2 into carbonates.

3. Quasi-Periodic Climate Change. State-of-the-art orbital models predict significant variations in the eccentricity, precession, and obliquity of Mars. These variations are predictable only for the past 10 My. Beyond that time, orbit parameters become chaotic. Obliquity variations have received most attention because they are large (0°-60° over the planet's history) and they determine the latitudinal distribution of solar insolation and, hence, the ultimate distribution of CO2 and H2O in the regolith-atmosphere-cap system. As the obliquity increases the following are expected to occur. Polar regions warm and equatorial regions cool. Any CO2 in the polar regolith would be driven into the atmosphere and surface pressures would rise. Models indicate that the increase in surface pressure would be less than 25 mb - if there is not a large reservoir of CO2 as ice or clathrate buried in the polar regions. If there is, an increase of 200 mb or so is plausible, but even this amount is not enough for significant greenhouse warming. Ice could become globally stable at the surface whereas it is only stable at the poles today. Dust storms would occur more frequently due to the increase in atmospheric mass and the intensity of the solstice circulation. As the obliquity decreases the following are expected to occur. Polar regions cool, CO2 returns to the polar regolith, and eventually permanent polar caps form. At this point the planet transitions from a regolith buffered system to a cap buffered system. Much of the regolith CO2 would be transferred to the caps which could become quite large. Surface pressures could fall to 0.5 mb or less making CO2 no longer the main atmospheric constituent. Dust storms would cease and the caps would cold trap water desorbing from the regolith.


Vugraphs

VG. 1. Early Mars Greenhouse Models

VG. 2. Global mean surface temperature as a function of surface pressure for several values of the solar constant. Solid lines assume an albedo of 0.25; dashed lines an albedo of 0.10. From Pollack et al. (Icarus, 71, 1987).

VG 3. Fig. 3. Evolution of the atmosphere pressure ....... From Melosh and Vickerey (Nature, 338, 1989).

VG. 4. Chemical weathering lifetime of a CO2 atmosphere as function of surface pressure for three different temperatures. Adapted from Pollack et al. (Icarus, 71, 1987).

VG. 5. Fig. 4. Time scale for resupplying ...... From Pollack et al. (Icarus, 71, 1987).

VG. 6a. A schematic diagram ..... From Schaefer (Geochim. Cosmochim. Acta. 57, 1993).

VG. 6b. Variation of pressure, pH, .... From Schaefer (Geochim. Cosmochim. Acta. 57, 1993).

VG. 7a. Vertical temperature profiles ... From Kasting (Icarus, 94, 1991).

VG. 7b. Surface temperature versus surface pressure .... From Kasting (Icarus, 94, 1991).

VG. 8. Left panel: Evolution of various CO2 reservoirs as a function of time. Right panel: Surface temperatures as a function of time. From Haberle et al. (Icarus, 109, 1994).

VG. 9. Some Possible Solutions to the Early Mars Dilemma.

VG. 10. Ocean Induced Climate Change.

VG. 11. Same as VG. 7, but with the introduction of a 0.5 (top), 1.0 (middle), and 2.0 (bottom) bar "pulse" of CO2 at one billion years ago. These pulses are assumed to result from the formation of an ocean due to volcanic activity as described by Baker et al. (Nature, 352, 1991). Figure taken from Gulick et al. (Icarus, submitted, 1996).

VG. 12. Quasi-Periodic Climate Change.

VG. 13. Fig. 6. Same as Fig. 5 ..... From Ward (Kieffer et al., eds, Univ. Arizona Press, 1992).

VG. 14. Annual average insolation at the top of the atmosphere for several different obliquities as a function of latitude. Calculations assume present day solar luminosity and Mars eccentricity.

VG. 15. Fig. 6. Temperature in the regolith ..... From Fanale et al. (Icarus, 50, 1982).

VG. 16. Atmospheric pressure as a function of obliquity as predicted by the model of Fanale et al. (1982).

VG. 17. Fig. 2-6. Models of the stability of ground ice .... From Mellon and Jakosky (J. Geophys. Res. 1995).

VG. 18. Cartoon of the conditions on Mars at the extremes of obliquity. At low obliquity (left) permanent caps form, atmospheric pressure drops, dust storms cease, and water is cold trapped at the poles. At high obliquity (right) the caps vaporize, CO2 desorbes from the polar regolith, surface pressure rises, dust storms occur frequently, and ground ice can be stable to very low latitudes. From Kieffer and Zent (Kieffer et al. eds, Univ. Arizona Press, 1992).

VG. 19. Issues Regarding Q-P Climate Change.

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